Meeting MOS and meteograms
May 1, 2019
Sure, you check METARs and TAFs (terminal aerodrome forecasts) as part of your preflight planning, but there are other ways of presenting the weather that you might find appealing and useful. One—model output statistics (or MOS, pronounced “moss”)—uses numerical guidance that gathers observations from multiple sources, runs them through computer algorithms of the atmosphere, and publishes three-day textual forecasts for hundreds more airports than those having TAFs. The most common MOS forecasts use data from one of two computer models—the GFS (Global Forecast System) or the NAM (North American Mesoscale). MOS forecast information comes from twice-daily model runs and uses a different set of abbreviations and codes than TAFs, which come out four times a day and typically cover a 24- or 30-hour forecast period.
A model output statistics forecast for Marfa, Texas, using numerical (computer-generated) forecast guidance from the Global Forecast System model. WDR stands for wind direction, and WSP stands for wind speed. For visibility, the “7” represents visibilities greater than six miles; a “1” would be less than a half-mile. As for cloud height forecasts, the “8” indicates ceilings greater than 12,000 feet; a “2” would represent ceilings from 200-400 feet agl. The OBV stands for obstructions to vision, with the “N” meaning “none.” On the whole, the next couple of days looks like good VFR in Marfa. Find full descriptions of GFS and MOS abbreviations online
GFS and NAM MOS forecasts are available on the National Weather Service website for many stations, many of which are at general aviation airports not served by TAFs. (MOS forecasts also are available on the ForeFlight app, in easy-to-read text presented in an hourly format.) Remember, TAFs are valid for an area within a 5-statute-mile radius of the airport. MOS forecasts can be valuable advisories, but TAFs are the official sources for airport forecasts. That’s because they are created by meteorologists at the National Weather Services’ weather forecast offices who are familiar with local weather patterns. But, guess what they check when making their forecasts—yep, MOS data.
Don’t want to decode a bunch of text codes? Then meteograms can provide visual charts of weather conditions at airports and other, user-defined (by latitude and longitude) locations around the nation. Meteograms also use MOS data as source material but are not as detailed as MOS-generated forecasts, vary in design, and can extend to eight days in the future. Like MOS information, meteograms are not valid as official preflight weather sources, but they can offer plenty of high-quality situational awareness. Search around and you’ll find more websites offering meteograms of various designs, including the Rapid Update Cycle model’s output, which lets you access up to 600 airports. By comparing MOS text and meteogram forecasts you can see how the GFS, NAM, RUC, and other sources can come up with different ideas of future weather. Comparing them against TAFs can shed insights on how aviation meteorologists buy into them—or not.
Thomas A. Horne
AOPA Pilot Editor at Large
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.